On Aug 30, 2013, at 9:14 PM, subramanya sadasiva <[email protected]> wrote:
> Hi Barry, > The linear solvers converge in 1 iterations. But the SNES solver fails when I > use the direct solver. Hmm. I don't have an explanation for this. Could you send some output when running with -snes_monitor -ksp_monitor_true_residual for both the iterative case and the direct solver case? Barry > Thanks, > Subramanya > > > Subject: Re: [petsc-users] Behaviour of Newton Methods with Direct Solvers. > > From: [email protected] > > Date: Fri, 30 Aug 2013 21:11:36 -0500 > > CC: [email protected] > > To: [email protected] > > > > > > I would not expect this. Are you sure that direct solvers are actually > > working well? You can run -ksp_type gmres -pc_type lu to "accelerate" the > > direct solver. Under normal circumstances the direct solver will converge > > in one iteration but if, for example, the problem is singular or nearly > > singular several iterations might solve the linear system but one may not. > > > > Barry > > > > On Aug 30, 2013, at 9:08 PM, subramanya sadasiva <[email protected]> > > wrote: > > > > > Hi, > > > Is there a reason that the convergence of the newton methods in SNES is > > > much better with iterative solvers instead of with direct solvers? This > > > is particularly in reference with the VI solvers. I get nearly quadratic > > > convergence with the iterative solvers, but the code just diverges or > > > converges linearly when i run with -ksp_type preonly . > > > Thanks, > > > Subramanya > >
